The invention relates generally to the assessment of the condition of dynamoelectric machines. More particularly, the invention relates to a visual inspection-based method and apparatus for measuring the tightness of retaining ripple springs in dynamoelectric machines, particularly electric generators.
Dynamoelectric machines such as electric generators include a rotor and a stator. Rotors are generally constructed from a steel forging and include a number of slots that run the length of the rotor. Rotors are electrically wound by placing conductors referred to as rotor windings into the slots of the rotor.
Stators are generally constructed from a number of stacked, metal laminations. Stators also include slots, which run the length of the stator. Stators are electrically wound by placing conductors known as stator coils into the armature slots of the stator.
Conventional stator coils are frequently held in place in stator slots using a retention assembly such as a stator wedge assembly including a stator wedge, a top retaining ripple spring, and a shim. In this configuration, a stator coil is placed into an armature slot, a shim is placed above the stator coil, a top ripple spring is placed above the shim, and a stator wedge having a beveled edge is driven into a groove near the head of the armature slot, securing the stator coil, the shim, and the top ripple spring. The top ripple spring provides compressive force to keep the stator coils held firmly in the armature slot.
Over time, stator wedges may become loose. If a stator wedge becomes loose, it can permit a stator coil to vibrate, which can cause catastrophic failure in an electric generator. In order to avoid such vibration, it is desirable to periodically inspect the tightness of the ripple springs. Such inspections present a challenge, because ripple springs are difficult to access within a generator and are concealed by the stator wedge.
There are a number of conventional approaches to inspecting the compression of ripple springs. One approach involves manually tapping the stator wedges. Another approach involves measuring the depth of the surface of ripple springs through pre-formed test holes in the wedge. A third approach involves physically displacing the wedge and measuring the resulting wedge movement.
There are significant challenges associated with the conventional approaches to testing ripple-spring tightness. The first approach, manually tapping stator wedges, is extremely subjective. The results vary greatly between different inspectors.
The second approach, using a depth gauge to take measurements through pre-formed test holes, is time consuming and is only possible in generators having stator wedges with pre-formed test holes. Many generators do not have such pre-formed test holes. In order to use this method on existing generators without test holes in the stator wedges, the units must be rewound using wedges with access holes.
The third approach, physically displacing the stator wedge, involves impacting a stator wedge and then measuring the displacement of the stator wedge with a sensor such as an optical or capacitive sensor to give an indirect indication of the compression of the ripple spring beneath the stator wedge.